They'll probably work out an extension about 10 days before the season starts, but we all know WR is probably one of the most important positions in terms of needing reps and establishing chemistry, so I'd imagine he'll be quite rusty when he comes back (and possibly very disgruntled if a deal doesn't get done).
Lamb has said several times how it took him a while to get going after sitting out everything last year, so even if he does come back he probably won't be the version we saw last year until October or November.
Not great for a team with a razor thin margin for error based on how last year went.